1. Field of the Invention
The present invention relates generally to solid state lasers, and more particularly, to a liquid-cooled solid-state laser structure.
2. Description of Related Art
Neodymium-doped yttrium lithium fluoride (Nd:YLF) is a very attractive laser material for many medium-to-high average power diode-pumped solid-state laser applications due to its long fluorescence lifetime, natural birefringence, intermediate gain, and low thermal lensing. Unfortunately, Nd:YLF has a low thermal stress fracture limit which heretofore has limited its use in commercial and military systems. Part of the stress fracture problem in liquid-cooled Nd:YLF lasers comes from within the flow channel. As the laser rod expands under a thermal load, stresses around the rod in the vicinity of an O-ring that seals the cooling jacket can cause the rod ends to fracture. As a result, the laser crystal is destroyed and the coolant can escape from the flow channel and contaminate other optical elements within the laser.
Currently, post-fabrication procedures are being developed to strengthen the surface of Nd:YLF rods. These procedures, which involve multiple etch and polish steps, are designed to remove the microcracks at the surface of the crystal which are believed to be the cause of the low thermal stress fracture limit in the Nd:YLF rods fabricated by conventional methods. Unfortunately, the improved stress fracture characteristics achieved using such strengthening procedures are difficult to preserve as the rods are handled during transport, manufacturing assembly, and maintenance. This is because handling the rods can result in scratches, digs, chips, and other surface damage which serve as nucleation sites for large-scale fracture when the rod is placed under mechanical and thermal stress. Furthermore, erosion of the rod surface by the cooling fluid may produce weak spots which can also serve as nucleation sites for fracture.
There is no known prior art directly related to the relief of mounting stresses in medium-to-high average power liquid-cooled solid-state lasers. U.S. Pat. No. 4,637,028 entitled "Conductively Cooled Laser Rod," and U.S. Pat. No. 4,969,155 entitled "Integrating Laser-Diode-Pumped Laser Apparatus", which are assigned to the assignee of the present invention, use (1) a sapphire (Al.sub.2 O.sub.3) insert to conduct the heat from the laser rod and (2) an elastomer to suspend the laser rod. Both patents use a conductive interface between the sapphire insert and the surrounding metal heat sink to remove the heat from the sapphire insert. This conductive cooling approach is well suited for smaller size rods operated at low to medium power levels. However, for large rod sizes and/or high average power operation, liquid cooling is the preferred cooling method.
It would therefore be an advantage in the solid state laser art to have a laser structure that reduces thermally-induced mounting stresses on low fracture strength solid-state laser media and is compatible with liquid cooling.